1. Field of the Invention
This invention relates generally to a surface acoustic wave filter device, and is directed more particularly to a surface acoustic wave filter device in which ripples in the pass band are suppressed by means of a step type transducer.
2. Description of the Prior Art
In general, a surface acoustic wave filter device can produce a coherent ultrasonic wave and detect the same by an interdigital transducer of the transversal type. This device may be used as a linear phase band pass filter or a low ripple band pass filter which serves as a band pass filter for an intermediate frequency signal in an FM radio receiver or as a band pass filter for a video intermediate frequency signal in a television receiver. From a practical point of view, however, various echo components occurring therein become obstacles and hence cause ripples in the frequency characteristics and phase characteristics in the pass band. Distortion in phase is essentially due to multiple reflection echos between inputs and outputs of the surface acoustic wave filter.
There are two components in the reflection echo of the surface acoustic wave on the interdigital transducer. One of the components is an electrical reflection due to acousto-electric regeneration from a terminal load, and the other component is a mechanical reflection due to the acoustic wave impedance mismatch between metallized and unmetallized sections of the transducer.
In order to cancel such a multiple reflection echo, the finger electrodes of an interdigital transducer are divided into a plurality of electrode elements or transducer elements in the beam width direction, which is the direction perpendicular to the beam propagation direction of a surface acoustic wave, to form a plurality of surface acoustic wave propagation channels resulting in a phase difference .DELTA..theta. between adjacent channels. Accordingly, the echo, caused by the mechanical reflection due to the acoustic impedance mismatch between metallized and unmetallized sections of the transducer is suppressed (refer to the gazette of the Japanese Patent Application laid open No. 107156/1974).
With this prior method, one of the echo components can be suppressed, but other echo components cannot be suppressed and hence the multiple reflection echo can not be suppressed sufficiently. As a result, ripples appear in the frequency and phase characteristics of the pass band, and hence such a prior art method is not practical.